Lithium secondary batteries, which are light in weight and have a high energy density, are widely used commercially as a power source for portable devices. Currently, lithium secondary batteries are also receiving considerable attention as a possible power source capable of offering high output power for large-scale applications (e.g., a power source for vehicles) and are now under active development.
The electrolytes for use in lithium secondary batteries, however, are usually non-aqueous electrolytes prepared by dissolving a lithium salt in a non-aqueous solvent. Non-aqueous electrolytes have lower ionic conductivity than electrolytes comprising an aqueous solution. Accordingly, it is difficult to achieve lithium secondary batteries having high output power.
Under the circumstances, various techniques to achieve a high output power lithium secondary battery have been proposed. For example, Japanese Laid-Open Utility Model Publication No. Sho 63-133065 and Japanese Laid-Open Patent Publication No. Hei 5-121064 propose a spirally wound design lithium secondary battery (i.e., a lithium secondary battery in which a strip-shaped positive electrode and a strip-shaped negative electrode are spirally wound with a separator interposed therebetween) in which a current collecting lead is connected to a center portion of an electrode. By connecting a current collecting lead to a center portion of an electrode, it is possible to reduce the battery resistance, which results in a lithium secondary battery having increased output power.
In a battery having reduced battery resistance by connecting a current collecting lead to a center portion of an electrode, if an external short circuit occurs, a large short circuit current flows in the battery. The short circuit current concentrates on the current collecting lead, which heats up the current collecting lead. This may cause the adjacent separator to melt. If the separator melts, it causes an internal short circuit, which may lead to overheating of the battery.